Device-to-device, D2D, communication is a well-known and widely used component of many existing wireless technologies, including ad hoc networks. Examples include Bluetooth and several variants of the Institute of Electrical and Electronics Engineers, IEEE, 802.11 standards suite such as WiFi Direct. These systems operate in unlicensed spectrum.
Recently, D2D communications as an underlay to cellular networks have been proposed as a means to take advantage of the proximity of communicating devices and at the same time to allow devices to operate in a controlled interference environment. Typically, it is suggested that such D2D communication shares the same spectrum as the cellular system, for example by reserving some of the cellular uplink resources for D2D purposes. Allocating dedicated spectrum for D2D purposes is a less likely alternative as spectrum is a scarce resource and (dynamic) sharing between the D2D services and cellular services is more flexible and provides higher spectrum efficiency.
Devices that want to communicate, or even just discover each other, typically need to transmit various forms of control signalling. One example of such control signalling is the so-called (discovery) beacon signal, which at least carries some form of identity and is transmitted by a device that wants to be discoverable by other devices. Other devices can scan for the beacon signal. Once they have detected the beacon, they can take the appropriate action, for example to try to initiate a connection setup with the device transmitting the beacon. For certain communication modes (e.g. connectionless communication, typically employed for groupcast and broadcast transmission) the beacon signal might carry a scheduling assignment indicating the associated data transmission to potential receivers. Connectionless communication is typically a unidirectional communication mode that does not require acknowledged connection setup. Other forms of control signalling may be carried by the beacon channel, too.
It may also be desirable to support D2D operation for out of network coverage User Equipments, UEs. In such case, different synchronisation options are possible: UEs may synchronise to a global reference (e.g. a Global Positioning System, GPS) which is in general different from the synchronisation reference of deployed networks. Possibly, UEs may operate in a fully asynchronous fashion (no synchronisation reference, at least for discovery). A further option is that clusters of UEs synchronise to a specific UE (in the following called a Cluster Head, CH) which provides local synchronisation to its neighbour UEs. Different clusters are not necessarily synchronised. It may further be desirable to support for inter-cell discovery scenarios where UEs camping on possibly unsynchronised cells are able to discover each other.
Additionally, operating D2D between different cells (that may happen to be unsynchronized or have large propagation delays) may require direct synchronization between the UEs participating in the D2D communication.
In order to detect possibly unsynchronised beacons and to perform channel estimation, each beacon is provided with DeModulation Reference Signals, DMRSs, if the radio transmission technology employs Long Term Evolution, LTE. DMRSs in each beacon are mapped to one or more OFDM symbols. Each DMRS is generated from a known sequence with good autocorrelation and cross correlation properties (e.g. such sequences are derived from Zaduff-Chu sequences).
Even though various options are possible, a possible solution for the mapping of beacons to the radio frames is to multiplex the beacons from different UEs in the frequency domain (Frequency Division Multiplex, FDM) within selected subframes.
Beacons are typically characterised by narrow bandwidth (e.g. 1 Physical Resource Block, PRB, corresponding to 12 subcarriers in LTE). Obtaining reliable estimation of the timing and Doppler shift associated to each beacon even at the low Signal to Noise Ratio, SNR, required for beacon detection implies a significant DMRS overhead in each beacon. Such overhead reduces the number of resource elements potentially allocated to the beacon's payload and consequently increase the beacons code-rate, affecting the coverage of the beacons.